Function, aesthetics, and comfort are prime objects of restorative dentistry, and the ability to treat occlusal problems is directly related to the ability to capture and transfer jaw motion to the restoration. Existing conditions of the patient must be analyzed by means of a functional method and the mere relation of the teeth to each other is not enough. Unless the relationship satisfies the functional requirements of the whole mouth, it may become a destructive function, creating a metabolic demand in excess of the normal requirements of the mouth.
To achieve these objectives, and to prevent trauma, wear and loss of centric relation contacts, the cuspal elements must be free to travel in and out of centric relation closure and free to perform their functions without interference or lateral stresses. The height, position, and relation of the cuspal elements are determined by studying the determinants of occlusion, namely, ridge and groove direction determinants (axis-orbital horizontal plane), cusp height and fossae depth determinants (sagittal plane), and determinants of the lingual concavity of upper anterior teeth. Since the length or height of the cusps is influenced by the condyle path, and the position and form of the cusps are determined by the Bennett movement which must harmonize with the lateral shift, it is necessary to observe the interferences between centric relation and centric occlusion position and the cusp pathways during eccentric mandibular movements to determine the formation of corrective measures. See Evaluation, Diagnosis, and Treatment of Occulusal Problems by Peter E. Dawson (1974) and Volume One: Mouth Rehabilitation Clinical and Laboratory Procedures by Max Kornfeld (2nd Ed. 1974).
Because the musculature motivates the movement of the mandible and the temporomandibular joint determines the movements, the determinants of the mandibular movements that dictate the occlusal morphology must be obtained and reproduced correctly to properly treat abnormal occlusion. Unless these varieties of motions can be accurately recorded and reproduced through laboratory registrations of the study casts of the teeth, occlusal corrections may become necessary.
Laboratory duplication includes making study casts of the maxillary and mandibular teeth. Registrations are taken of the jaw movements of the patient by one instrument and are later transferred to a second instrument. The casts are then mounted to the second instrument to the correct axis and the second instrument is used to simulate the movements of the patient's jaws in the laboratory to permit the review of the occlusal relationships of the patient's teeth. Various prior art instruments have been used to record and attempt to duplicate jaw movement including the pantograph, stereographic instruments, articulators, and relators.
The pantograph graphically records the various positions and movements of the mandible and then the resulting graphs are utilized to adjust an articulator which attempts to reproduce the paths of the patient's movements. The pantograph relies upon the hinge axis even though the hinge axis has control only along the border paths when we masticate. The pantograph has two facebows with six recording slides and six styli that scribe the paths of the centers of rotation of the mandible. The hinge axis is first located, and then an innerocclusal record is taken at centric relation or centric occlusion. The pantograph then graphically records the movements of the jaw. In making these records, the clutches mounted to the pantograph have to be separated vertically so that there will be no tooth guidance during the registrations, and at the same time separated a minimum distance to reduce the error. After a record of the jaw movements has been registered and the orbital guide set, the arms of the pantograph are locked together and the entire assembly is removed intact from the patient and is transferred to the mounting frame or jig that holds the facebow in proper relation to the adjustable articulator while the attaching stone sets.
These are two types of articulators, the semiadjustable articulator and the adjustable articulator. The semiadjustable articulator is often referred to as the checkbite articulator which reproduces the horizontal condyle paths from an innerocclusal record made at centric relation and a bite record made in the protrusive position. The resultant path is a straight line between the two points. Lateral pathways are set from the centric bite record (innerocclusal record) and records are made in the left lateral and right lateral jaw positions. The semiadjustable instrument can accurately record the hinge axis but cannot record the full range of lateral and protrusive condylar movements. Further, semiadjustable instruments do not precisely record the Bennett shift. With adjustable articulators, a more complete preoperative occlusal analysis is possible than is possible with semiadjustable articulators. The improvement in accuracy is the difference between straight line vs. curve pathways plus the differences of timing of the Bennett shift.
A stereographic instrument may be used in place of the pantograph for taking registrations which are subsequently transferred to another instrument such as the TMJ Articulator or the Gnathic Relator. The stereographic instrument generally includes upper and lower intraoral clutches having a central bearing point therebetween. An innerocclusal record is made in centric relation or centric occlusion. Recordings are then made by indenting three or four points, mounted on one clutch, into doughy self-curing acrylic on the surface of the opposite clutch and then moving the mandible. When the stereographic record is completed, the acrylic guide paths are allowed to set hard. In using the TMJ Articulator, the stereographic clutches are mounted on the TMJ Articulator using the centric bit record (innerocclusal record). A doughy mixture of TMJ acrylic is placed in plastic receptacles at the condyles and the condyles are guided by moving the points through the paths made by the indentations. After the condyle paths are recorded, the casts are mounted in place of the clutches. The Gnathic Relator is a device that uses the stereographic clutches after anterior guidance has been corrected in the mouth. The clutches are screwed into the study casts thereby permitting the border pathways to be reproduced by hand using the handheld models. Further description of the above is found in Evaluation, Diagnosis, and Treatment of Occulusal Problems, by Peter E. Dawson (1974) at pages 124-131 and is incorporated herein by reference.
Many practical problems have been encountered using such prior art techniques and instruments. Pantographic tracings are no more accurate than the paths of movement that the operator records. Many pantographic errors result from failure to achieve a true terminal hinge condyle position during any part of the tracing. Further, there is the fear of recording a protrusive lateral path instead of the straight lateral border path.
Another disadvantage of pantographic devices is that the tracings must be made at a considerably opened vertical dimension to make room for the clutches. It is essential that the terminal hinge axis be recorded precisely or the incorrect axis of closure will introduce errors. It is also probable that in some mouths, at least, the border movements are different at the opened position from what they are at the correct vertical position.
Errors in mounting of pantographic devices are common and easy to make. The slightest movement of either clutch produces a magnified error at the tracing plate. Studies have shown that reproducibility of pantographic tracing is seldom achieved.
One of the most important purposes of an articulator is to relate the upper and lower models to the correct horizontal axis. Semiadjustable articulators do not permit a correct relationship to the axis of closure. Sizeable errors are introduced into all aspects of occlusal form when incorrect horizontal and vertical axes are not used. The problem with simple hinged type articulators is that the only movements that they can make are movements the patient cannot make.
Tooth relations on the articulator will not be the same as in the mouth unless the opening-closing hinge in centric relations are the same on the articulator as in the patient's mouth. The articulator must reproduce the same relation of the casts to the axis of the instrument that the teeth have to the axis of the mandible. Unless the casts can be occluded by closing them on the same arc of closure as that exhibited by the patient, erroneous conclusions may be drawn concerning existing patterns and the need for restorative procedures for the patient.
The stereographic instrument requires intraoral clutches which must be fitted inside of the patient's mouth. The central bearing point must be properly sized or the vertical dimension may be in error. No recordings are made other than in the closed position since an open position will cause the pointers to lift from the acrylic. Further, if the intraoral clutches are not mounted in the mouth in the same position as on the study casts, error will be introduced.
Both the pantographic devices and stereographic instruments require an innerocclusal record made in centric relation or centric occlusion. Such an innerocclusal record is required to accurately orient and relate the maxillary and mandibular casts on the articulator. Unless the casts are properly oriented, upper to lower, on the articular, the recordings cannot be properly reproduced. It has been found to be difficult to obtain an accurate bite in centric relation or centric occlusion.
Transfer is also required using the stereographic instrument. The TMJ articulator relies upon the accurate transfer of the registrations of the stereographic instrument. Any time such a transfer is made and recordings have to be duplicated, errors arise.
A great amount of time is often wasted on procedures that have little or no value in specific cases. The articulator should be used to achieve precise reproduction of condylar pathways and not become subservient to the particular techniques such as the transfer of the recordings to a duplicator of the jaw movements.
Mandibular movements are three-dimensional in character and have been found to be complex. The envelope of motion has anterior, posterior, and lateral limits, and there is a sequence of motion from one position to the other. Not only do the joints rotate in the vertical, horizontal, and sagittal planes, but they are capable of bodily side shift (the Bennett movement) which adds another motion to the complexity of movement. When rotation and translation are combined, there is no constant axis of control. The axis that changes is called the instantaneous axis of rotation, and it is only in control for an instant and can only be located when motion is stopped. If the instantaneous centers cannot be located, there is no way of plotting the functional envelope of motion and the border paths of the complete envelope must be used. The task of duplicating such mandibular movements has been found to be lacking in prior art devices.
The present invention overcomes the deficiencies of pantographs, stereographic instruments, articulators and relators by eliminating the necessity of an innerocclusal record at centric relation or centric occlusion and by eliminating the transfer of the recordings to a mechanical duplicator of the movements of the jaw. Further, the present invention provides a three-dimensional recording which is more repeatable and readable than those that exist in the prior art. The present invention becomes a substitute for the jaws in which all occlusal determinants and the opening and closing axis relations are precisely incorporated. The present invention does not require the location of the hinge axis or the establishment of the axis-orbital plane or centric relation. No interpolation is required in its recordings and duplication of jaw movement. The present invention permits reproduction of the patient's mandibular movements, accurate record and check of centric relation, alteration of the vertical dimension with certainty, recordation of the limit of the maximal occlusal opening, minimization of the adjustment in the mouth during restoration, and duplication of all arcs of closure. Other objects and advantages of the present invention will become apparent from the following description.